Adds a way to store and retrieve a default VAO for OSX (needed for
a weird bug where you can't link a shader without being attached to a
vao...). Adds a new demo in the OSX Xcode project for showing how to do
pingponging between FBO textures (a cellular automata demo). Adds a new
method in Texture.cpp to let you replace and update all the data in a
Texture.